The invention relates generally to the sensing of cardiac signals, and more particularly to a system for readily identifying the P wave portion of the signal.
There are two major pumping chambers in the heart, the left and right ventricles. Simultaneously contracting, these chambers expel blood into the aorta and the pulmonary artery. Blood enters the ventricles from the left and right atria, respectively. The atria are smaller antechambers which contract in a separate action that precedes the major ventricular contraction by an interval of about 100 milliseconds (ms), known as the AV delay, approximately one-eighth of the cardiac cycle. The contractions arise from a wave of electrical excitation which begins in the right atrium and spreads to the left atrium. The excitation then enters the atrio-ventricular (AV) node which delays its passage via the bundle of His into the ventricles.
Electrical signals corresponding to the contractions may be displayed on an electrocardiagram. A small signal known as the P wave accompanies atrial contraction while a much larger signal, known as the QRS complex, with a normally predominant R wave, accompanies the ventricular contraction. Repolarization prior to the next contraction is marked by another small signal in the electrocardiagram known as the T wave. Reliable detection of the P wave is greatly desirable because the P wave is an extremely useful timing signal for devices that artificially stimulate the heart muscle.
For example, one of the problems treated by cardiac pacers is heart block caused by impairment of the ability of the bundle of His to conduct normal excitation from the atrium to the ventricle. It has long been apparent that in treating this form of heart disease, it is desirable to base the stimulation of the ventricles on the unimpaired P wave cycle. This synchronization maintains the heart's normal physiological pacing pattern. Thus, the sino-atrial node, which governs the interval between atrial depolarizations (i.e., the atrial rate) according to the body's needs, controls the artificial ventricular rate in the normal manner.
The functioning of so called physiological pacing systems based on atrial timing depends on the accuracy of intracardiac detection of the natural P wave. AV sequential pacers operating in the double demand (DDI) mode, for example, also depend on P wave discrimination for inhibiting unnecessary and potentially detrimental atrial stimulation.
For these and other reasons, the ready identification of a P wave in intracardiac systems is desirable.
In an article entitled "Characteristics of Intracardiac Electrograms II: Atrial Endocardial Electrograms" by Victor Parsonnet et al., appearing in the journal Pace, Vol. 3, p. 406 (July-August 1980), it has been proposed that the differences between amplitude and slew rates of P and R waves recorded from the atrial appendage should allow sensing amplifiers to easily distinguish them by means of appropriate filter design.